Biochar: a new type of carbon sink?

In the global fight against climate change, conserving carbon sinks across the world, such as the Amazon rainforest, is critical. A carbon sink is something that absorbs more carbon dioxide from the atmosphere than it releases and many carbon sinks around the world do this in staggering quantities, locking it up in organic material such as trees, soil, shrubs, and mosses. However, once they are damaged, many carbon sinks such as rainforests or peat bogs, are typically incredibly difficult to repair fast enough to significantly mitigate the damage done. Not only does the damage result in emissions including methane and CO², but there is also a loss of the area’s associated benefits for local biodiversity and nature. In addition, natural carbon sinks are predicted to become less and less effective the longer humanity continues to emit greenhouse gases.

But what if we could create our own carbon sinks? What if we could sequester carbon for a significant number of years within components of the structures we build? And what if, by doing so, we could encourage the use of a material that could play a significant part in sequestering more and more carbon in the very buildings we live and work in?

Through locking in carbon previously sequestered in timber production, biochar has the potential to become individual carbon sinks in each use case. Biochar is created through subjecting timber waste to high temperatures in a low-or zero-oxygen environment. It locks in the carbon previously sequestered in the timber production phase that would otherwise be emitted back into the atmosphere through burning or composting.

However, there are specific concerns that must be addressed in each use case. Any use of biochar must be part of a stewardship approach, otherwise the net result is an increase in emissions. Without replacing the original feedstock, biochar production would just involve destroying trees, among other timber sources, and consequentially destroying those trees’ ability to sequester carbon.

The energy used in the production of biochar is also a key variable. Any energy-intensive process will naturally incur significant emissions if it isn’t carried out in the most energy-efficient manner with the highest amount of renewable energy possible. The world is already seeing the benefits of centring energy-intensive production processes in places with access to significant sources of renewable energy.

For example, aluminium production (typically one of the most carbon-intensive construction materials) by Hydro in Karmøy, Norway has a cradle-to-gate carbon intensity of 3.06 kgCO²e / kg and 4.23 kgCO²e / kg for their CIRCAL and REDUXA aluminium, respectively, compared to an average value of 42.71 kgCO²e / kg for the average generic aluminium sheet in the German market. Concentrating biochar production and utilisation in locations such as these could significantly reduce the lifecycle emissions incurred by the production of biochar.

Novocarbo also provides a good example of how to carry out low-carbon production of biochar. The waste heat energy produced by their biochar production plants is fed into district heating grids and can also be deployed to individual production sites. They manufacture economically viable products, which can be used in the construction industry, in horticulture and landscapes as a soil conditioner and in green infrastructure approaches to stormwater management. Their operations also provide an excellent example of biochar’s sequestration potential - they’ve sequestered over 2,000 tCO² in 2023 alone. Multiple academic studies have also found biochar to have significant sequestration potential when it is added to soils, acting as a catalyst for the creation of humus and soil absorption of atmospheric CO².

As with all material supply chains, using low-carbon transport options (chiefly by cutting out all air freight) will be key towards ensuring low A5 transport emissions.

Deciding on the best species of vegetation to use can be reduced to several variables: time taken for the plant to grow; the kgCO2e sequestered by the species; and other ancillary variables such as biodiversity value, climate resilience, and usable crops. The quicker the species grows, the faster the feedstock can be used, and the more biochar that can be created. The more CO²e sequestered by a species used for biochar, the more carbon-negative the production of biochar would be (assuming that feedstock is replaced, as previously described above).

Finally, any green infrastructure, such as trees, can offer significant additional benefits: biodiversity and nature, usable crops, and climate resilience to name but a few. These benefits must be balanced with the aforementioned others – if a certain species can provide critically needed support to local biodiversity, then it may be of greater value than one which sequesters more CO²e. Certain forestry techniques may also be utilised to ensure that a whole tree isn’t used at once, thereby safeguarding the original feedstock.

Biochar is also an excellent potential way to promote biophilic design through using natural materials for whole structural elements, such as a building’s external façade, rather than individual plants. Just considering its inclusion at an early design stage would drive the whole design process towards a more biophilic direction and make designers consider other options. Increasing our use of natural materials throughout the built environment will be key in our fight against climate change, so increasing awareness of this can only have a positive effect.

Biochar could also become a key material in decarbonising our construction practices; it is a key example of the sort of step change we absolutely have to take to achieve meaningful action. The remaining time to stop irreversible climate change and limit global warming to 2.0°C, let alone the 1.5°C agreed in the Paris Agreement of 2015, is ticking down with every day we do not make enough progress. Incorporating biochar in our designs could potentially significantly reduce the embodied and whole life carbon of what we build, and at Cundall we look forward to doing so as much as possible.